1. Field of the Invention
The invention relates to the telemetry of downhole data in a measurement while drilling system, and more particularly, to a method and apparatus for the transmission of acoustic data and the filtration of acoustic noise within a stream of flowing drilling fluids.
2. History of the Prior Art
In the oil industry, receiving data from downhole sensors during a drilling operation provides information which is of great value to the drilling operator. Such data transmissions may generally be referred to as being part of a "measuring while drilling" (MWD) system. Downhole measured parameters such as weight on the bit, fluid pressures, fluid temperatures, formation nature, gamma ray measurements and accelerometer data indicative of the inclination of the drill stem adjacent the drill bit all vary with time. These parameters are of great interest for effecting the formation of the borehole in the most efficient and economical manner and their transmission is thus a critical feature of the drilling operation.
Many different prior art techniques have been proposed for effecting the telemetry of data downhole. Such information is generally measured by sensors located near the drill bit and relayed to the surface in order to make the data readily available for analysis during the drilling operation. The telemetry, or relay system, is thus an integral part of the operation and a myriad of telemetry techniques have been employed. For example, it has been proposed to utilize the metal drill string as a carrier for both acoustic and electrical signals as well as the flow conduit for drilling fluids. Such drill string communication links carry digitally encoded information from within the borehole to the surface well head. It has been established that of all these techniques, the use of acoustic pressure pulses imposed upon the column of flowing drilling fluids within the drill string has proven to be the most effective transmission medium for data relay of monitored downhole parameters.
It is conventional in the prior art to supply a stream of drilling fluid into the borehole by relatively large pumps located at the well head. The drilling fluid, or mud, is pumped under pressure down the central opening in the drill string at the well head to force the mud through the string and out apertures located in the bit. This flow cools and lubricates the bit and carries off pieces of the formation cut by the bit during the drilling operation. The mud flows back to the surface in the annular space between the outer walls of the drill string and the sides of the borehole. At the well head, the mud is routed by conduit from the mouth of the borehole to a fluid storage pit and/or mud processing equipment located at the surface. Such equipment may include degassing units and mud filtration systems which prepare the fluids for subsequent conveyance downhole.
Drilling fluid is conventionally forced down into the drill string by means of large reciprocating piston pumps. Such units must generally have a capacity for moving from 600 to 1,000 barrels of fluid per hour down into a borehole and back out again. For this reason, great force is needed and the pressure impulses generated in the column of drilling fluids by the reciprocating circulation pumps are quite large. The pumping action thus creates a very noisy acoustical environment within the drilling fluids. Such noise obviously interferes with the relatively low level transmission of acoustic data pulses of a downhole telemetry system utilizing the drilling fluid as a transmission medium. In addition, the high pressure acoustic pulses generated by the pumps are also reflected from each discontinuity in the flow path. Such discontinuities occur where the various sections of conduits are coupled for directing fluids into and out of the borehole. It may thus be seen that acoustic data signals transmitted from within the borehole and which are to be received and analyzed by receiving transducers located at the well head are virtually buried within a large quantity of acoustic noise. The transmission signals must therefore be extracted from the background noise before the borehole data can be analyzed to provide useful information to the drilling operator.
Various prior art techniques have been proposed for reducing the acoustic noise level in the drilling fluid stream to aid in the reception of data. For example, one technique is shown in U.S. Pat. No. 3,488,629 wherein pump noise impulses are filtered from the fluid line by simultaneously supplying the impulses to both inputs of a differential pressure detecting meter. The simultaneous receipt of pump pressure pulses is caused by two equal path lengths for pressure communication from the pump. However, the differential pressure detecting meter has two unequal pressure path lengths as seen from the borehole side. This is effected simply by meter location within the meter input flow line. In this manner, pressure pump impulses cancel one another but downhole transducer impulses produce a differential output signal. A similar technique is disclosed in U.S. Pat. No. 3,716,830 which teaches cancellation of both mud pump pulses as well as conduit and impedance mismatch reflections thereof by applying received signals from two acoustic transducers through a differential amplifier. One of the transducer signals is phase shifted corresponding to the delay time in the reflected signal to cancel both mud pump pulses and unwanted reflections thereof to thereby isolate acoustic pulses from the downhole transducer.
The aforesaid prior art techniques specifically address and are necessarily dependent upon the geometry of the fluid flow system and transducer spacing therein. A particular flow geometry must be maintained in order to successfully eliminate acoustic noise from the drilling fluid flow path for improvement of the reception of acoustic data signals from downhole. Drill string and pumping configurations vary, however, and many prior art preprogrammed filtration patterns can quickly become out of phase and cannot be automatically calibrated. It would be an advantage to provide a system for filtering of acoustic noise from the drilling fluid flow which is independent of specific geometrics and specific transducer spacings. Moreover, it is desirable to provide a noise filter system which is universally applicable to any fluid flow stream used as an acoustic transmission line for improving the signal to noise ratio of acoustic data transmitted thereby.